The present invention relates generally to an exposure apparatus, and more particularly to an exposure apparatus used to manufacture various devices including semiconductor chips such as ICs and LSIs, display devices such as liquid crystal panels, sensing devices such as magnetic heads, and image pickup devices such as CCDs, as well as fine patterns used for micromechanics. The present invention is suitable, for example, for an immersion type exposure apparatus that immerses, in the fluid, the final surface of the projection optical system and the surface of the object to be exposed, and exposes the object through the fluid.
Conventionally employed reduction projection exposure apparatuses use a projection optical system to project or transfer a circuit pattern on a mask or a reticle onto a wafer, etc., in manufacturing such a fine semiconductor device as a semiconductor memory and a logic circuit in the photolithography technology.
The critical dimension transferable by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Smaller resolution has recently been required with a demand for the finer processing to the semiconductor devices. Therefore, in addition to use of the exposure light with a small wavelength, the projection optical system is expected to improve the resolution by using a higher NA. At present, the projection optical system has accelerated an increase of its NA, and it is expected to develop the projection optical system having a NA of 0.9 or greater.
On the other hand, the light sources for the exposure apparatus have changed from a KrF laser (with a wavelength of 248 nm) to an ArF laser (with a wavelength of 193 nm). Currently, developments of an F2 laser (with a wavelength of 157 nm) and EUV (with a wavelength of 13.5 nm) are promoted for the next generation light sources.
With this background, an immersion exposure has attracted attentions as a method that uses the ArF laser and the F2 laser for more improved resolution. See, for example, Japanese Patent Application, Publication No. 10-303114. The immersion exposure arranges the fluid as a medium at a wafer side of the projection optical system (or an image surface side), and promotes a higher NA. Specifically, the projection optical system's NA is n·sin θ where “n” is a refractive index of the medium, which can be increased up to “n” by filling the medium (fluid) having a refractive index greater than that of the air, i.e., n>1, in at least part of the space between the projection optical system and the wafer. In other words, the immersion exposure improves the resolution by increasing the projection optical system's NA viewed from the wafer side up to 1 or greater.
On the other hand, in order to align the reticle with the wafer during the exposure, the exposure apparatus includes plural alignment optical systems. The alignment optical system is roughly classified into two types, i.e., an off-axis alignment optical system that detects an alignment mark on the wafer and uses it for the alignment for the wafer, and a through the reticle (“TTR”) alignment optical system that detects, via the projection optical system, a position of the alignment mark on the wafer (or wafer-side reference plate provided on a wafer stage), which is referred to as a wafer-side pattern and corresponds to a reticle-side pattern that is an alignment mark on the reticle (or reticle-side reference plate provided on a reticle stage). The TTR alignment optical system is also referred to as a through the lens (“TTL”) alignment optical system.
Since the immersion type exposure apparatus fills the fluid in the space between the projection optical system and the wafer so as to implement the NA of 1 or greater, there is no imaging relationship between the reticle-side pattern and the wafer-side pattern in the TTR alignment optical system. As a result, the light intensity detecting method for detecting the light intensity using a light intensity sensor provided on the wafer stage, and then a positional relationship between the reticle-side pattern and the wafer-side pattern, can neither image the reticle-side pattern on the wafer-side pattern nor precisely align the reticle-side pattern with the wafer-side pattern. On the other hand, an image detecting method that images an alignment mark on an image pickup device cannot image the wafer-side pattern on the image pickup device via the projection optical system, or align the reticle-side pattern with the wafer-side pattern.
The light intensity detecting method has an area that has a refractive index of 1, such as the air and vacuum, between the wafer-side reference plate and the light intensity sensor. When reticle-side pattern is imaged on the wafer-side pattern by using the light having the NA greater than 1, the light having the NA greater than 1 is totally reflected on the back surface of the wafer-side reference plate, which back surface opposes to the pattern surface, and does not reach the light intensity sensor. Therefore, a correct measurement value cannot be obtained due to offsets of the measurement values and the deteriorated measurement reproducibility. On the other hand, the light having the NA smaller than 1 is not totally reflected on the back surface of the wafer-side reference plate, but the reflectance becomes higher due to the large incident angle. Therefore, disadvantageously, the light having a high NA is reflected on the back surface of the wafer-side reference plate and its light intensity incident upon the light intensity sensor is smaller than that of the light having a small NA. The image detecting method that requires the illumination light to enter the back surface side of the wafer-side reference plate cannot use the incident light having a NA greater than 1, due to the area that has a refractive index of about 1 between the wafer-side reference plate and an emitting section that emits the illumination light.
A non-immersion type exposure apparatus has a similar problem that the light having a large NA is reflected on the back surface of the wafer-side reference plate, due to the higher NA used for the projection optical system. In order to receive the light having an arbitrary NA, a sensor needs to have a large area and, when a light intensity sensor having a large area is provided on the wafer stage, the wafer stage becomes too large.